1. Technical Field
The present invention relates to a technique of applying a drive signal to and thus driving a capacitive load such as a piezoelectric element.
2. Related Art
There are a number of piezoelectric elements driven by application of a predetermined drive signal, such as an ejection head installed in an inkjet printer. To drive such piezoelectric elements, usually a drive waveform signal is power-amplified and then applied as a drive signal to the piezoelectric elements.
As a method for power-amplifying a drive waveform signal, for example, a method using a class-D amplifier is known (JP-A-2005-329710 and the like). In this method, power amplification is carried out after a drive waveform signal is pulse-modulated and thus converted to a pulse wave-like modulated signal. Both a pulse width modulation (PWM) system and a pulse density modulation (PDM) system can be applied as pulse modulation systems, but pulse width modulation is usually used. Then, after the resulting pulse wave-like modulated signal is power-amplified and thus converted to a pulse wave-like modulated signal that changes between a power-supply voltage and a ground (amplified digital signal), a modulation component is eliminated by a low pass filter, thus generating an amplified drive waveform signal (drive signal).
Here, a high frequency having a margin in relation to a frequency band of the drive signal is set as a cutoff frequency of the low pass filter so that elimination of a signal component of the drive signal can be prevented during the elimination of the modulation component by the low pass filter. Moreover, in order to enable complete elimination of the modulation component resulting from the pulse modulation by the low pass filter, a high frequency having a margin in relation to the cutoff frequency of the low pass filter is set as a carrier frequency at the time of the pulse modulation. Consequently, in the class-D amplifier, for example, a carrier frequency that is higher than the frequency band of the drive signal by ten times or more is used.
However, if the frequency band of the drive signal includes a high frequency band, there is a problem that a very high frequency must be set as the carrier frequency when power amplification is to be carried out using the class-D amplifier. For example, a drive signal of an ejection head installed in an inkjet printer has a frequency component of 500 kHz or higher and therefore a high frequency of 5 MHz or higher needs to be set as the carrier frequency. This has adverse effects, for example, operation of a switching element cannot catch up or increase in power loss for switching lowers power efficiency. Although using a high-order low pass filter may be possible in order to obtain sharp filter properties, there are problems not only that the configuration of the low pass filter becomes complex, but also that high-order transmission properties may cause a strain in the drive signal in relation to high frequency bands and fluctuations in load.